A number of processes require the grinding of material using many types of apparatus to grind different kinds of materials. One such grinding apparatus is an imp mill, which is a particular type of hammermill. The imp mill is one form of pulverizer commonly employed for reducing the size of minerals, organics and chemicals. One of the earliest uses to which imp mills were put was that of the pulverization of coal, and particularly in those applications wherein it was desired to pulverize the coal for direct firing. Imp mills are also widely used in the complete processing of such products as organic insecticides, soya flour, starches, litharge for storage batteries, phosphate materials, synthetic resins, potassium compounds, clay materials and in literally dozens of other applications in which precision grinding and drying are an important part of the production process.
Imp mills generally have a plurality of hammers suitably attached to a row of disks, which in turn are attached to a rotor shaft, of which are housed within a cylindrical grinding chamber. The grinding chamber has an air inlet and an air outlet disposed to allow forced air to pass through the grinding chamber and carry pulverized material (i.e., coal) of a desired size out of the imp mill. Each row of hammers includes a plurality of hammers disposed circumferentially around a corresponding disk or pair of adjacent discs. The hammers may be fixed rigidly or pivotally pinned to the disks. As the rotor and disks are rotated by a motor, material is fed into one end of the grinding chamber. The rotating hammers crush and pulverize the material as the material progresses through the grinding chamber. The dimensions of the disks and hammers, number of hammers, rotor speed, the flow rate of the air through the grinding chamber, and the dimensions of the grinding chamber determine the particle size exiting the outlet of the imp mill.
In a normal operation of an imp mill, the first row of hammers where the material feeds in functions as pre-crushers. In other words, the first row provides the grinding and crushing of the initial particles, which are the larger particles. The first row of hammers is therefore subject to more severe wear than the other rows of hammers. Consequently, the hammers in the first row wear quicker than the rest of the hammers. The premature wear of the first row of hammers results in a shorter life cycle for all the hammers because all the hammers are typically replaced at the same time as the replacement of the first row hammers. Another disadvantage of the premature wear of the worn first row of hammers is the resulting vibration of the mill rotor due to rotor imbalance cause by uneven wear with certain hammer configurations.
A need therefore arises for an imp mill that provides a relatively consistent or uniform wear across all the hammers of the mill, particularly the premature wear of the first row of hammers. The uniform wear of the hammers results in a consistent grind (i.e., particle size) throughout the life of the hammers. Further, the reduced wear of the first row of hammers results in less hammers being changed, and thus longer hammer life, less down time, and less time spent changing the hammers. Uniform hammer wear will also maintain rotor balance and thus avoid vibration due to uneven wear. Such an imp mill results in lower total hammer, labor, and maintenance costs, as well as lower unit energy consumption.